1. Field of the Invention
The present invention relates to a photopolymerizable dental composition which is cured by irradiation with visible light, and a photopolymerization initiator used therefor. More particularly, the present invention relates to a dental composition which causes less change of a color tone before and after curing, and also can be cured by irradiation with light in a wide wavelength range. The composition of the present invention is used for fillers, adhesives, surface coating materials, restorative materials for dental crowns, cements, etc.
2. Description of the Related Art
Current dental treatments view aesthetic qualities as one of the most important elements. There is a strong requirement for the characteristics of dental materials to be such that the color of restorations resembles that of a natural tooth, and has a beautiful white color.
In actual treatment, a color tone is determined by comparing a color tone of the natural tooth of the patient with that of materials such as fillers. However, materials comprising a combination of a conventionally used visible light-photopolymerization catalyst and a polymerizable monomer displayed inferior aesthetic qualities since a color tone varies before and after curing, resulting in poor compatibility with a color tone after mounting in the oral cavity.
In the dental field, a visible light-polymerizable resin has been widely used. The disclosure of UK Patent No. GB1408265 resulted in broad application of a photopolymerization initiator therefor including a hydrogen abstracting initiator comprising an a-diketone compound and an amine compound, such as camphorquinone having a maximum absorption wavelength of 470 nm. However, a photopolymerization initiator comprising an a-diketone compound and an amine compound has problems such as drastic yellowing and change of a color tone of the cured article associated with the amine compound, and poor thin-layer surface curability.
(Bis)acylphosphine oxide compounds disclosed in U.S. Pat. Nos. 4,265,723, 4,298,738, 4,792,632 and 5,965,776 show excellent photopolymerizability in an ultraviolet or near ultraviolet range and are therefore widely used in the photopolymerization industry field. The (bis)acylphosphine oxide compounds are less likely to cause yellowing of the cured article and also display excellent internal curability. Therefore, (bis)acylphosphine oxide compounds are used for photocuring of transparent thick films and materials containing pigments having a large hiding power, and have also found recent application in the dental field. However, since the absorption wavelength range of (bis)acylphosphine oxide compounds is on the short wavelength side of visible light, the (bis)acylphosphine oxide compounds have the drawback that curing occurs when a photopolymerization device with a light source such as a halogen lamp is used, while curing does not occur when the device having a narrow photoradiation wavelength range such as blue LED is used. A dental material using (bis)acylphosphine oxide compounds has a drawback that it has a colorless color tone before curing but yellowing occurs after curing.
Japanese Unexamined Patent Publication (Kokai) No. 2000-16910 reports that a curable composition containing a (bis)acylphosphine oxide compound and an amine compound displays physical properties of cured articles such as a high strength and a low amount of an unpolymerized material on the surface. However, this initiator has a problem that severe change in a color tone of the cured article is caused by the amine compound.
Japanese Patent No. 3,442,776 proposes a visible light-polymerizable adhesive comprising an a-diketone compound, a (bis)acylphosphine oxide compound, an amine compound and a polymerizable monomer. However, a curable composition using an amine compound as the photoinitiator has a problem that severe change in a color tone of the cured article occurs and a usable life drastically decreases.
Japanese Patent No. 2,629,060 reports a photopolymerizable dental surface coating material comprising dipentaerythritol hexaacrylate and a (bis)acylphosphine oxide compound. This patent reports that the photopolymerizable dental surface coating material is excellent in thin layer surface curability since it contains dipentaerythritol hexaacrylate and a (bis)acylphosphine oxide compound. However, there is a problem that curing caused by dipentaerythritol hexaacrylate and the (bis)acylphosphine oxide compound causes browning of the cured article.
Japanese Unexamined Patent Publication (Kokai) No. 6-55654 proposes a photocurable dental crown material comprising polyfunctional (meth)acrylate, an alumina filler and acylphosphine oxide. Although discoloration of the material is improved by this invention, surface curability of a thin-layer is not taken into consideration.